A typical motor vehicle manufactured today has a number of safety systems which function to deploy airbags, and possibly to retract seatbelts. The number of airbags provided in a vehicle continues to increase. Systems now in development deploy multiple airbags to protect vehicle occupants from front, rear, and side impacts, and to position the vehicle occupant's body to withstand acceleration. Deployment of safety systems requires sensors that can detect and characterize a crash as it occurs. Wide use of advanced active safety systems is dependent on producing systems that are sufficiently cost effective that they may be economically employed on a large number of vehicles.
Typically the lowest cost sensors are those formed on an integrated circuit chip using the technology used to form electronic circuitry. This technology is used to fabricate micro devices or accelerometers that can detect accelerations indicative of a vehicle crash. These sensors are particularly cost effective when the sensor can be fabricated together with the deployment logic circuitry using the same technology that is cost effective for large scale integration. However the very small size of these devices makes them sensitive to electromagnetic interference and the like, which can result in false indications of a crash.
Thus an important role remains for macro scale mechanical devices which are less prone to false readings. Such devices are used to verify the occurrence of an actual crash. These macro scale devices employ a sensing mass mounted on a spring or pendulum. Motion of the mass is detected by actuation of a reed switch or a magnetic field sensor of the Hall type.